1. Field of the Invention
The invention relates to the technical field of motion estimation and, more particularly, to a method for fast macroblock mode decision.
2. Description of Related Art
Current video coding standards essentially use a motion estimation (ME) to remove redundancies in an image sequence and thus achieve high image compression efficiency. In the H.264/MPEG-4 AVC video coding standard, a variable blocksize motion compensation is used to improve the coding efficiency. In general, there are 16*16, 16*8, 8*16 and 8*8 blocksize modes for a macroblock (MB) level and 8*8, 4*8, 8*4 and 4*4 blocksize modes for a sub-macroblock (sub-MB) level.
In conventional techniques, the motion estimation and the macroblock mode decision are concurrently performed. For each blocksize, the motion estimation is first performed, and a resulting motion cost is used for the mode decision, which relatively increases computational amount.
To overcome the problem of dramatically increasing the computational amount of a macroblock decision, U.S. Pat. No. 6,782,052 has disclosed a solution to compute a mode frequency prediction for a current block in accordance with the mode frequencies of blocks neighboring the current block. As shown in FIG. 1, block E is the current block, and its neighboring blocks are blocks A, B, C and D. In this case, block modes mA, mB, mC and mD are determined for the blocks A, B, C and D respectively and next their frequencies FmAFmBFmCFmD are determined. Thus, the mode frequency prediction (F0) is computed and represented as α×min(FmA, FmB, FmC, FmD) where α is smaller than one and greater than zero. If a mode m has a frequency Fm smaller than the prediction (F0), the mode m is eliminated. The macroblock mode decision can reduce some modes, but the computational amount for such a decision is still large when a current block and its neighboring blocks are related low so as to relatively reduce the efficiency.
Therefore, it is desirable to provide an improved macroblock mode decision method to mitigate and/or obviate the aforementioned problems.